1. Field of the Invention
This invention relates to a semiconductor fabrication method, and more particularly to a method of formation for a via hole with a wet etching process.
2. Description of Related Art
In general, there are two typical processes used to form a via opening. They are a wet-dry etching process and a full dry etching process.
FIG. 1A and FIG. 1B are cross-sectional views schematically illustrating a conventional fabrication flow of formation of a via opening by a wet-dry etching process. In FIG. 1A, the wet-dry etching process first starts, for example, from a metal layer 10, which includes, for example, three sub-layers of Al/Ti/TiN. Then, an oxide layer 20 is formed over the metal layer 10, and a photoresist layer 25 is formed over the oxide layer 20. The photoresist layer 25 is to be used to pattern a via opening 30. The via opening 30 is formed by performing an isotropic wet etching process on the oxide layer 20. Next, in FIG. 1B, an anistropic dry etching process is further performed to completely form the via opening 30, which exposes the metal layer 10. During the dry etching process, some residual oxide (not shown) usually is deposited on the bottom of the via opening 30. In order to remove the residual oxide, a further etching process, called an over-etching process, is performed. The metal layer 10 is therefore etched out a little as shown in FIG. 1B. Even though this over-etching process can remove the residual oxide material, the Ti/TiN material on the top surface of the metal layer 10 is inevitable consumed, which causes the aluminum material to be exposed. This method increases probability that aluminum is eroded and increases an aspect ratio (AR) of the via opening, in which the AR is a ratio of the depth to the width.
Another conventional method of formation of a via opening is to use the full dry etching process. This conventional method uses only dry etching and is similar to the previous method except that it skips the wet etching in FIG. 1A. Since the dry etching is still used, an over-etching process is needed to remove the residual oxide material. This causes the same problems as those seen in the previous conventional method.